The closest known state of the art is German Patent No. 24 08 414, which discloses an apparatus for testing the stereoscopic vision. A similar vision-testing apparatus is also disclosed in the "Zentralblatt fuer Arbeitsmedizin, Arbeitsschutz, Prophylaxe und Ergonomie"(Industrial Medicine, Industrial Safety, Prophylaxis and Ergonomy"), Volume 30 (1980), Pages 110 to 113. This vision-testing apparatus has a concave mirror, at a focal point of which is a test object. A semi-translucent dividing mirror is arranged in the beam path between the test object and the concave mirror, onto which semi-translucent mirror looks the eye of the person tested. This vision-testing apparatus is supposed to enable viewing of vision objects in free space. Through suitable use of the apparatus it is possible to view a test object in free space at any desired distance from infinity to a near point given by the focal distance of the concave mirror. Relatively natural viewing conditions result hereby with respect to the accommodation and convergence for the visual distance of the person tested from infinity to a short distance of approximately five centimeters. Only a single test object is thereby used, the virtual image of which can be projected at any desired distance. The adjustment to various distances is thereby done by moving the test object relative to the combination of partly translucent mirror and concave mirror. The significant disadvantage resulting from this is that both the concave mirror and also the dividing mirror are arranged directly in the vicinity of the eyes of the person tested. Thus, the test object is viewed under the same optic conditions which exist when a magnifying glass is arranged directly in front of the eye of the person tested and when the viewed object is moved relative to the magnifying glass in order to illustrate different optic distances. The individual visual signals appear thereby, depending on the adjusted distance, at different angles. Thus, different resolving power values result with the adjustment of the same visual signals. The larger disclosed visual signals can be more easily recognized by the person tested and simulate a better resolving power. This adulterates in a nonrecheckable manner the testing of the nearsightedness of the person tested.
German Patent No. 32 41 958 discloses an apparatus for carrying out vision tests in which two different optic paths exist, which can be utilized on one hand to test nearsightedness and on the other hand to test farsightedness. When testing the near vision, the optic path does not have a focusing mechanism. The disadvantage of this apparatus is that it must be of a relatively large construction and requires a considerable mechanical input. Furthermore, it is not guaranteed that the viewing direction of the person tested is adjusted exactly to the desired viewing angle, so that the possibility of incorrect tests exists.
Vision-testing devices of the above-mentioned type are utilized to quickly examine the visual functions of the eyes of a person tested. Measurements can be made of the binocular and monocular resolving power of the eye, of the heterophory, of the stereoscopic vision, and of the color vision. Furthermore, such vision-testing devices are also advantageous for testing nearsightedness, hyperopia or presbyopia.
The known vision-testing devices are mostly derived from a simple stereoscope in which the person tested looks through two collective lenses arranged at a fixed distance from one another. The person tested can see test figures or other objects which are arranged in duplicate, each in the focal plane of the lenses. The test images of the test figures, which must be identical, are in this manner optically at infinity so that the pair of eyes of the person tested can merge the two images into one unit. From the fixed arrangement of the two collective lenses result theoretically the same optic viewing relationships, but through the aberation of the lenses, interferences in the binocular vision can occur. A further disadvantage of these known arrangements is that the person tested looks into a dark box, in which the brightly illuminated test figures or test objects are arranged. These are optically reproduced to infinity, but the person tested has, due to the arrangement of the test object inside the box, always the impression of closeness which adulterates the recognition by the person tested. In other words, because the box itself is small, the person looking into it expects to see a test object located close to his or her eyes rather than a test object located (due to optics) at infinity, and therefore when both eyes are being used the person will have a mental tendency to automatically move his or her eyes to the "cross-eyed" state which is normally used to view a close object and which involves highly convergent lines of vision from the respective eyes. This tendency is referred to as pyschical convergence or mental convergence. This too can cause interferences in the monocular or binocular vision, which are expressed by blurred images or double images. A further disadvantage is that, with this type of testing of the resolving power with test objects having optically the same size, different results are obtained in comparison with an ophtalmologic testing of the resolving power at a five or six meter distance.
In addition to the psychical convergence, which is caused by the above-mentioned interferences in the test reading, the instrument nearsightedness plays a role in such viewing devices and causes blurred vision when viewing figures optically shown in infinity. The closeness of the test object is simulated in such devices by inserting additional lenses and prisms between the fixed viewing lenses and the test object. This stimulates the pair of eyes to accommodate and converge, which again results in new sources for error. Due to the limited dimensions of the testing apparatus, only a specific number of such additional lenses and prisms can be used, so that the number of testing distances to be adjusted is seriously limited.
A basic purpose of the invention is to provide a vision-testing apparatus of the above-mentioned type, which with a simple design and with simple reliable operation produces a plurality of vision-testing possibilities, and with a compact design enables a person to be tested with a constant viewing angle.